As you say adjoining accommodation does not have your problem and assuming the two are essentially the same construction and generally similar (neither has been greatly modified) then the difference is almost certainly down to different lifestyles. Builder has pointed to the likely headlines.
We had tenants in a property which suddenly was damp after three decades in our ownership without problems. The reason came down to heavy loading of internal atmosphere with water from a) drying washing indoors, b) a lot of boiling of assorted things (vegetarian diet, magic health drinks etc.), c) lack of ventilation, etc.
Heat well and continuously along with ventilating regularly and the problem should reduce or go away - do not dry things on radiators, cook without ventilating the kitchen, do not shower or bathe without ventilating adequately during/afterwards.